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This paper presents results of tests of fracture toughness of concrete modified with the addition of class F fly ash (FA), covering all three models of cracking: I, II, and III. Knowing the values of critical stress intensity factors: ${\rm{K}}_{\rm{Ic}}^S$, K IIc K IIIc generalized fracture toughness Kc was determined. The tests were carried out on concrete composites with a different content of FA, i.e.: 0% (FA-00), 20% (FA-20) and 30% (FA-30). The tests were performed out after 28 days, i.e. the standard period of curing of the specimens. Based on the obtained results it can concluded that the class F FA additive in the amount up to 30% of mass of cement clearly changes the fracture toughness of concrete, at all models of cracking. Concrete with a 20% addition of FA – FA-20 is characterized by the most favorable mechanical parameters, whereas composite with a 30% addition of FA – FA-30 had low compressive strength and generalized fracture toughness. Moreover, the results of fracture toughness tests, are convergent qualitatively with compressive strength results.
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